Many of the commercially available laser printers, as well as some of the recently introduced electronic copiers, include flying spot raster output scanners (ROS's) for printing latent electrostatic images on xerographic photoreceptors. These photoreceptors generally have steeply sloped contrast vs. exposure characteristics (high gamma), together with well defined exposure thresholds (called the "xerographic threshold"), so they characteristically yield high contrast, bitmapped images (e.g., black and white). Some xerographic printers operate in a "write black" mode to optically expose the image foreground for printing by means of an "exposed area development" process, while others operate in a "write white" mode to expose the image background for printing by means of a "charged area development" process.
As is known, both write black and write white xerography are suitable for color printing. So-called "full color" xerographic prints customarily are composed by printing three or four different color separations (e.g., cyan, magenta and yellow for three color printing, and cyan, magenta, yellow and black for four color printing) in superimposed registration on a suitable substrate, such as plain paper. Highlight color prints, on the other hand, can be produced by printing as few as two color separations (e.g., black and a selected highlight color). There is, however, a common thread because each of these color separations generally is a high contrast image. It, therefore, will be evident that the fundamental operating principles and functional advantages of this invention apply to both monotone and color xerography.
Many of the ROS's that have been developed for xerographic printing employ a single beam or a multi-beam laser light source for supplying one or more intensity modulated light beams, together with a scanner (such as a polygon scanner) for cyclically deflecting the modulated laser beam or beams across a photoreceptor in a "fast scan direction" while the photoreceptor is being advanced simultaneously in an orthogonal, "process direction." In practice, each of the laser beams typically is brought to focus on or near the photoreceptor surface to provide a substantially focused "scan spot." The scan spot or spots, in turn, scan the photoreceptor in accordance with a predetermined scan pattern because the fast scan deflection of the laser beam or beams vectorially sums with the process direction motion of the photoreceptor. Indeed, the scan pattern is dependent upon and is determined by the scan rate (scans/sec.) of the scanner, the number of scan spots that are employed, and the process speed (inches/sec.) of the photoreceptor. Such a scan pattern produces an exposure pattern because the scans are superpositioned on the photoreceptor, regardless of whether the scans simultaneously or sequentially expose the photoreceptor. Accordingly, it is to be understood that the present invention applies to printers and other displays that employ single beam or multi-beam ROS's, even though this disclosure features the single beam/single scan spot case for the sake of simplification.
Laser illuminated flying spot ROS's ordinarily are designed to provide generally circular or elliptical scan spots. To a first approximation, such a scan spot is characterized by having a gaussian intensity profile (as is known, this may be a very rough approximation if the scan spot is truncated). Prior laser printers generally have employed scan patterns that are selected to have a scan pitch (i.e., the center-to-center displacement, in the process direction, between spatially adjacent scan lines) that is comparable to the diameter of the scan spot as determined at an intensity level that is equal to one-half of its maximum or peak intensity. This sometimes is referred to as the full width, half max. ("FWHM") diameter of the scan spot.
Images often contain many transitions. For instance, black and white and other dual tone images have transitions at the boundaries between their foreground features and their backgrounds, such as the transitions that demark line edges, font contours, and halftone dot patterns. Color images commonly include still additional transitions at the boundaries between differently colored foreground features. Consequently, the perceived quality of monotone and color prints tends to be strongly dependent upon the precision with which the printing process spatially positions these transitions.
Modern laser xerographic printers typically are designed to print at spatial resolutions ranging from about 300 dots/inch ("d.p.i") to about 600 d.p.i. As a practical matter, the image transition positioning precision of these printers can be increased to an extent by increasing their spatial resolution, but the frequency responses of the photoreceptor/developer combinations that currently are available for xerographic printing usually impose an upper limit on the resolution that can be achieved. Moreover, even when increased resolution is technically feasible, the additional resolution imposes further and potentially burdensome requirements on the optical and electrical design requirements of these printers, so there usually is a cost/performance tradeoff to be considered. Specifically, the cost of xerographic print engines tends to escalate as their spatial resolution is increased because of the additional memory and bandwidth these printers require for faithfully rendering higher resolution bitmap images without sacrificing throughput.
In apparent recognition of these technical challenges, others have proposed template matching techniques for more precisely controlling the size, positioning and number of picture elements ("pixels") that are printed on xerographic photoreceptors to render bitmapped images. For example, template matching has been developed for reducing the severity of certain printing artifacts, such as the observable stairstep-like scan structure (commonly referred to as "jaggies") that sometimes degrades the xerographically printed appearance of non-vertical and non-horizontal lines. See Tung U.S. Pat. No. 4,847,641, which issued Jul. 11, 1989 on "Piece-wise Print Image Enhancement for Dot Matrix Printers" and Walsh et al U.S. Pat. No. 4,437, 122, which issued Mar. 13, 1984 on "Low Resolution Raster Images." Template matching effectively overcomes some of the sampling errors that are caused by the use of input data that is too coarse to accurately represent the higher spatial frequency content of the image. It does not, however, solve the control problems that are encountered in existing printers because of the significant non-linearity of the way in which the spatial positioning and profiles of the transitions in the images they print tracks changes in the intensity of the transitional boundary scans. This "addressability response" issue is discussed in greater detail hereinbelow.
Therefore, it is evident that there still is a need for simplified methods and means for enabling optical printers and other display systems that render bitmapped images on high gamma, photosensitive recording media, such as xerographic laser printers, to spatially position transitions within the images they render with a sub-resolution spatial precision (i.e., a precision that is greater than the spatial resolution of the system). More particularly, relatively inexpensive and easily implemented methods and means are needed for enabling laser ROS-type printers to spatially position transitions in the images they print to a precision, in the process direction, that is a sub-multiple of the scan pitch. It is also evident that, even if it is possible to spatially position transitions in an image with a subresolution spatial precision, on a high-gamma, photosensitive recording medium, there can still be errors or distortions in the image due to non-ideal operating conditions. These distortions, such as banding, misregistration and bowing, arise from the limits of mechanical precision which can be built into the printer. These limits arise from economic as well as technological considerations. Accordingly, relatively inexpensive and easily implemented means and methods are needed for enabling laser ROS-type printers to correct and/or avoid these distortions.